2018 Potato Postharvest Processing Evaluation Report
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Postharvest Processing Evaluation of Alaska Grown Potatoes A Specialty Crop Block Grant Project Introduction Potatoes have long been a staple produce of Alaskan agriculture. Between the years 2009-2016 Alaska growers have produced between 130,000 to 155,000 cwt annually amounting to over 2 million dollars in sales each year (2017 Alaska Annual Bulletin). There has been increasing interest in the use of Alaska Grown potatoes for processing in the local chipping and restaurant market, but this effort hasn’t been supported with data on the processing quality of our locally produced potatoes. To better meet the needs of the food service industries and to promote a growing market for producers, the Alaska Plant Materials Center (PMC) undertook a postharvest evaluation on our collection of potato varieties grown on site in Palmer, Alaska. The results of this research present timely and relevant data to Alaskan growers, processors and consumers. On a national level, the processing industry accounts for nearly 60% of potatoes produced annually. This trend has caused potato breeders to select for processing qualities, and quite a few processing cultivars have been recently registered and released for use. Although some of these newer varieties are grown here in Alaska, they have not been evaluated and compared to the data collected by growers in other regions or compared to established varieties that are known to do well here. Even if the physical qualities of the varieties were comparable to those grown elsewhere, Alaska is unlikely to compete in the national processing market because of our distance from any commercial processing facility and the small “family farm” scale of operation. On a local level, there is a favorable perception of using Alaska Grown produce that may help encourage Alaska Grown products to show up on menus across the state. That interest can only be sustained if quality products are produced and processed. The Alaska PMC maintains a large collection of potato varieties. Several cultivars known for processing were added for this experiment including Clearwater Russet, Tundra, Lelah and Sage Russet, to name a few. The inclusion of varieties developed and used for processing added an interesting dimension to the project because the growing conditions in Alaska are significantly different from those in other potato producing states. Alaska typically experiences a short 4-month growing season with long daylengths and reduced soil temperatures compared to other regions. We also don’t struggle with water shortages and are likely to receive excessive rainfall, especially as we near harvest in the autumn. Physiologically, the types of differences routinely encountered like varying specific gravity measurements and sugar concentrations are important factors affecting the processing potential of potatoes. Methods & Materials The entirety of the Alaska PMC maintenance collection is grown in the field annually as a matter of routine. This provides an opportunity to examine the collection for varietal purity, symptoms of disease or any anomalies, it provides a reservoir of tubers should there be a problem with the tissue culture collection and it also provides the opportunity to gather data on the varieties produced. This allows us to see natural variations in tuber characteristics as they fluctuate between growing seasons. Data on the specific gravity and tuber shape, color and size of the varieties maintained by the Alaska PMC has been collected and recorded since the 2014 field season. Degrees Brix measurements were taken in 2016 & 2017. From this data, 103 cultivars (See Appendix 1 for a complete list of the 103 screened cultivars) were selected for processing evaluation. Criteria used for selection was primarily a specific gravity higher than 1.080 or a recommendation from the potato breeder for the cultivars use for processing. The degrees Brix measurement was also considered but was given less weight because the results proved to be highly variable. The 2016 field season was used to add new varieties to the Alaska PMC collection and gather baseline data for the project. Data collected included tuber shape, size and color, specific gravity and degrees Brix. For comparison sugar concentration was also measured with a glucose strip and with a diabetes test kit. The glucose strip and the diabetes test kit were quickly discarded as a measure of sugar levels in the tubers because the results were consistently as high as the test could read or out of range making the expensive tests meaningless for comparison purposes. The 2017 potato field was planted and maintained as usual. Through an operational oversight, the first 90 varieties planted did not receive any fertilizer. The tubers were smaller than usual, and yield was decreased for the affected potatoes as expected. Of the 15 varieties included in the final evaluation, Peter Wilcox, Atlantic, Allagash and Krantz did not receive any fertilizer. The tubers were planted May 30-31, 2017 and were evaluated weekly starting 5 weeks after planting. Chemical vine desiccation occurred 15 weeks after planting at 105 days, and harvest occurred 17 weeks after planting on September 26-27, 2017. In that time, we experienced 711 Growing Degree Days (GDD) according to the Alaska Climate Research Center as measured at the Palmer Airport. We also had several wind storms which damaged the vines of many varieties and likely affected yields and tuber quality as well. The top growth of the 90 field varieties that were not fertilized were markedly small and pale compared to the rest of the field although the specific gravity from the tubers was not notably inconsistent with that measured in other years when all the varieties were fertilized. At harvest a 25-pound bag of each variety was collected and stored at 50°F with 99-100% humidity for 10-14 days. The temperature was slowly reduced over the following 2-week period to a holding temperature of 38°F and a holding humidity of 98%. After the tubers had equilibrated, a subsample of each of the 103 selected varieties weighing between 3.3-6.6 pounds was pulled and washed. This subsample was used to measure specific gravity which is determined by the formula; Specific gravity = __________weight of tubers in air______________ (weight of tubers in air) – (weight of tubers in water) A Martin Lishman Digital Potato Hydrometer was used to obtain the specific gravity. Results were compared with specific gravity measurements from previous years as that data was available i.e. cultivars new to the Alaska PMC collection had fewer years of data available. Additionally, a degrees Brix measurement was taken using an Atago Pocket Refractometer Pal-1. In December 2017, the selected varieties were removed from 38°F storage, cut with a Redco InstaCut Series 15000 with a 3/8” (1 cm) screen and fried at 375°F for 3 minutes with a Pitco Economy Gas Fryer in canola oil. The material was not rinsed or pre-prepared in any way. The fries were compared to the USDA “Color Standards for Frozen French Fried Potatoes” (Fifth Edition, 2007), assigned a color rating and photographed (See Figures 1 and 2). A panel of volunteers tasted the fries and shared their assessment which was noted. Taste was used as the most influential selection criteria and the top 15 preferred varieties were selected for the next stage of evaluation. On April 3, 2018, the 15 selected cultivars were removed from 38°F cold storage and placed at room temperature, approximately 60°F, and allowed to undergo reconditioning for 14 days. The philosophy behind reconditioning is that at warmer temperatures the respiration rate of a tuber will increase, and it begins to convert the reducing sugars glucose and fructose back into starch thereby decreasing the sugar concentration in the tuber. Reducing sugars react with available free amino acids during frying via the Maillard reaction and high levels of reducing sugars at processing result in unacceptably dark products (fries or chips) with an unappealing burnt taste. On April 17, 2018, after 14 days of reconditioning, a five-person panel evaluated each of the varieties after they were sliced 3/8” thick and fried at 375°F for 3 minutes in canola oil. A value between 1-10, with 1 being unacceptable and 10 representing a highly favorable critique, was assigned for the following qualities: Color, Flavor, Texture, Appearance and Overall (See Figure 3). The color score in this instance was the opinion of the panel as to whether the fry was an appealing color, it is separate from the color rating based on the USDA color score chart. The overall category was an independent assessment from the panel, it was not an average of the other 4 criteria. In addition to being evaluated by the panel, the fries were also assigned a color rating based on the USDA Color Standards chart (Fifth Edition, 2007) and photographed. The data was compiled and analyzed and was compared to the USDA fry color rating obtained in the previous evaluation. The comments from both evaluations were considered as well. From the data, each of the varieties are ranked by potential processing quality (See Figure 4). Results Figure 1: Data for the available specific gravity (SpG), annual °Brix, USDA Fry Color of tubers direct from cold storage (38°F), and USDA Fry Color of reconditioned tubers (60°F) presented for the 15 varieties selected for the final quantitative evaluation. SpG SpG SpG SpG °Brix °Brix FryColor FryColor Variety 2014 2015 2016 2017 2016 2017 (38°F) (60°F) Allagash 1.094 1.083 1.075 1.096 3.9 4.7 1 1 Atlantic 1.103 1.081 1.103 1.093 6.3 5.2 3 2 Bushes Peanut 1.121 1.098 1.108 1.081 4.2 4.8 3 4 Cowhorn 1.090 1.091 1.077 1.080 NA 4.5 3 4 Gui Valley 1.109 1.094 1.107 1.098 5.5 5.5 2 2 Krantz 1.088 1.082 1.090 1.075 5.4 4.7 4 4 Peanut 1.121 1.076 1.108 1.076 4.6 4.4 4 4 Clearwater Russet 1.076 1.079 5.4 6.3 4 3 Lamoka 1.097 1.092 5.3 6.9 4 3 Lelah 1.102 1.090 5 5.8 2 3 Sage Russet 1.055 1.087 1.065 4.6 3.8 4 4 Tundra 1.091 1.086 4.9 5.1 2 3 4 1.089 1.072 6.2 5.3 4 4 Peter Wilcox 1.085 1.098 4.7 5.3 4 4 Alturas 1.079 1.076 5.9 6.0 4 4 Reconditioning and the final fry evaluation occurred approximately 7 months after harvest.